1. Field of the Invention
The present invention relates in general to a printing apparatus having a print head for printing on a recording medium supported by a platen, and more particularly to adjustment of a head gap between the recording medium and the print head.
2. Discussion of the Prior Art
A printing apparatus generally has a platen for supporting a recording medium, and a print head to effect printing on the recording medium. Some printers are adapted to permit adjustment in the head gap, which is a clearance between the surface of the recording medium and the print head, depending upon the thickness of the medium. In a dot matrix printer using print wires, for example, a printing pressure between the print wires and the recording medium varies with the head gap. In an ink jet printer, the transfer of an ink material to the recording medium is affected by the head gap. Thus, the amount of head gap of the printer influences the printing result or quality of printed images. Since the head gap changes with the thickness of the recording medium, it is desirable to adjust the head gap to an optimum value for highest printing quality, when the thickness of the recording medium is changed.
In the light of the above, there is proposed a printer as disclosed in laid-open Publication No. 61-262161 of unexamined Japanese Patent Application. This printer includes a print head disposed movably in a transverse direction perpendicular to the length of a platen, a head advancing and retracting device for advancing and retracting the print head in the transverse direction toward and away from the platen, and head gap adjusting means for controlling the head advancing and retracting device, to adjust the head gap between the print head and the recording medium supported by the platen. The head gap adjusting means is adapted to first advance the print head for abutting contact with the recording medium supported by the platen, and then retract the print head by a suitable distance. Since the print head is retracted from the position at which the print head abuts on the medium, the head gap adjusted by the retraction of the print head reflects the thickness of the medium. The optimum head gap or the distance of retraction of the print head may be either a fixed value, or a variable which changes depending upon the thickness of the medium. In either case, the head gap can be suitably adjusted for excellent quality of the printed images.
The conventional printer capable of adjusting the head gap has either an automatic head gap adjusting arrangement wherein the adjusting device is automatically operated, or a manual head gap adjusting arrangement wherein the adjusting device is operated by the operator of the printer. However, the conventional printer does not permit both the automatic adjustment and the manual adjustment of the head gap. The automatic adjustment of the head gap assures sufficient printing quality if the recording medium is a generally used one and the printing is not conducted under special conditions. However, the head gap established by the automatic adjustment is sometimes inadequate and is preferably re-adjusted, if the recording medium is not a paper sheet or web, or the medium is a paper sheet or web made of a special material, or if the printing condition is otherwise special. This re-adjustment should be made by the operator, by using an operator-controlled adjusting device. Conventionally, however, the printer capable of automatically adjusting the head gap is not provided with operator-controlled means for permitting the operator to manually adjust the head gap or change the automatically established head gap.
Another problem experienced in the printer as disclosed in the above-identified publication is derived from the use of a stepping motor as a drive source for activating the head gap advancing and retracting device. The stepping motor is stepped in the forward direction to advance the print head toward the platen. After the print head is brought into abutting contact with the recording medium, the stepping motor is forcibly stopped even while stepping pulses are applied to the motor. Thus, the stepping motor undergoes an out-of-synchronization phenomenon upon abutment of the print head against the recording medium. This out-of-synchronization of the stepping motor is used to detect the abutting contact between the print head and the recording medium, and to reverse the operating direction of the motor, for retracting the print head away from the medium. Therefore, the printer suffers from vibrations and noises due to the abutment of the print head against the platen (medium) and resulting out-of-synchronization operation of the stepping motor.
The out-of-synchronization of the stepping motor used as the drive source of the head advancing and retracting device may be avoided by using a frictionally coupling clutch, which is adapted to transmit a drive force of the motor to the print head during movements of the print head, and undergo a slipping action upon abutment of the print head against the platen, thereby inhibiting the transmission of the drive force exceeding a preset upper limit. Since the clutch is brought to its disconnected state upon abutment of the print head against the platen, the stepping motor is protected against the out-of-synchronization phenomenon. However, the amount of operation of the stepping motor to bring the print head into abutment against the platen is set to be large enough to cause the clutch to be disconnected only after the print head has come into abutting contact with the platen, irrespective of a fluctuation in the initial position of the print head from which the print head is advanced for abutment against the platen. This arrangement inevitably suffers from a relatively long time of slipping of the clutch due to the continuing operation of the stepping motor after the print head has been stopped by the platen. Therefore, the life expectancy of the clutch tends to be shortened due to rapid wearing of the clutch.
An example of the conventional head advancing and retracting device is partly illustrated in FIGS. 16 and 17, in which reference numeral 101 designates a guide shaft for supporting a carriage 106 so that the carriage 106 carrying a print head 105 mounted thereon is slidably moved on the guide shaft 101 in the longitudinal direction of the guide shaft parallel to a platen 107. The guide shaft 101 is provided at its opposite ends with integrally formed eccentric support pins 102. The axes 04 of the eccentric support pins 102 are offset from the axis 03 of the guide shaft 101 by a radial distance .DELTA.l. The guide shaft 101 is rotatably supported at the eccentric support pins 102, by respective bearings 103 fixed to side walls 104 of the printer. The carriage 106 has a bearing metal 108 which is fitted on the outer circumferential surface of the guide shaft 101, so that the carriage 106 slides on the guide shaft 101, for reciprocating movements of the print head 105 parallel to the platen 107 when printing is effected on a recording medium supported by the platen 107.
When the eccentric support pins 102 are rotated by a suitable drive source such as a stepping motor as indicated above, the guide shaft 101 is rotated eccentrically with respect to the support pins 102, whereby the guide shaft 101 is displaced in the transverse direction, toward and away from the platen 107, over a range corresponding to the offset distance .DELTA.l. Thus, the power transmission mechanism illustrated in FIGS. 16 and 17 constitutes a part of the head advancing and retracting device for detecting the thickness of the recording medium and adjusting the head gap.
However, the outer sliding surface of the guide shaft 101 is exposed, and a foreign matter such as paper particles or dust may be deposited on the exposed sliding surface of the shaft 101, and may stick to the inner bearing surface of the bearing metal 108 of the carriage 106, while the carriage is reciprocating during a printing operation. Consequently, the friction force between the bearing metal 108 and the sliding surface of the guide shaft 101 tends to vary during use of the printer. More specifically, the foreign matter sticking to the bearing metal 108 increases the friction force, thereby increasing a resistance of the metal 108 to the rotation of the guide shaft 101 when the guide shaft 101 is rotated relative to the carriage 106 for detecting the thickness of the recording medium and adjusting the head gap. The increase in the above rotational resistance of the bearing metal 108 results in an accordingly increased force of abutting contact of the print head 105 with the platen 107 (recording medium). This fluctuation of the abutting force of the print head 105 with respect to the platen 107 adversely affects the accuracy of detection of the medium thickness and the accuracy of adjustment of the head gap.